I cast on the first one at 8 tonight and was done by about 10:30 while the NASCAR race was on. I’ve tried both single and doubled worsted-weight yarn and they worked for me, but I’d like to know if they’ll work for anyone else. 
This is meant to be a Girl Scout/church group/just for fun pattern, so let me know if it strikes you as difficult. I’d put up pictures, but I can’t find my camera tonight!
You need: size 10 1/2 needles
Pile of worsted yarn scraps at least 30" long; about two ounces should do for single yarn. The goofier the yarn looks, the better.
About one ounce of yarn in one piece to work toes
Cut yarn scraps to 30". Starting about 2" from the end of a scrap, cast on loosely until you have roughly 2" left. I got 24 stitches, but if my calculations are right, it doesn’t matter that much.
Tie another scrap onto the cast-on one, leaving about 2" of that one hanging out too, and knit across. Knot the ends at the end of the row. (If you like the dot-and-dash look of stripes in garter stitch, you can change colors every row; if you’d rather see solid stripes, make sure you work two rows with each color. I made a pair of pink and white slippers and one with rainbow stripes against white.)
Continue to knit across and tie off until you have 12 garter stitch ridges.
Now tie on the piece of intact yarn and knit two rows even.
On the next row, k2tog, then knit across.
Continue to k2tog at the beginning of every row until you have 12 stitches left. (K2tog) across.
Leaving an 8" tail, cut yarn, run cut end through the 6 remaining stitches, pull up to gather toe and knot securely.
With another scrap of yarn, stitch the heel seam. Now, if your feet are the intended recipients, put the slipper on and sew the toe seam until you get the fit you like around the ankle. The loose gauge and garter stitch make these ridiculously stretchy; a slipper 6" long when finished easily stretched to fit my 9 1/2" feet. Take the slipper off and flip it upside down so all the fringe hangs, then trim it if you think it’s needed.
Optional: If you crochet, you may like to grab the toe color and (dc in end of garter stitch ridge, ch 1) around. You can then run a length of ribbon or, better yet, elastic yarn through the eyelets for a better fit around the ankle.